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Neptunium ( , ) is a chemical element with the symbol Np and atomic number 93. A radioactive metallic element, neptunium is the first transuranic element and belongs to the actinide series. Its most stable isotope, 237Np, is a by-product of nuclear reactors and plutonium production and it can be used as a component in neutron detection equipment. Neptunium is also found in trace amounts in uranium ores due to transmutation reactions.

Characteristics

Silvery in appearance, neptunium metal is fairly chemically reactive and is found in at least three allotropes:
  • α-neptunium, orthorhombic, density 20.45 g/cm³
  • β-neptunium (above 280 °C), tetragonal, density (313 °C) 19.36 g/cm³
  • γ-neptunium (above 577 °C), cubic, density (600 °C) 18 g/cm³


Compounds

This element has four ionic oxidation states while in solution:
  • Np3+ (pale purple), analogous to the rare earth ion Pm3+
  • Np4+ (yellow green)
  • NpO2+ (green blue)
  • NpO22+ (pale pink)


Neptunium forms tri- and tetrahalides such as NpF3, NpF4, NpCl4, NpBr3, NpI3, and oxides of the various compositions such as are found in the uranium-oxygen system, including Np3O8 and NpO2.

Neptunium(V) fluoride, NpF5, is volatile like uranium hexafluoride.

Neptunium, like other actinides, readily forms a dioxide neptunyl core (NpO2), which readily complexes with carbonate as well as other oxygen moieties (OH, NO2, NO3, and SO42−) to form charged complexes which tend to be readily mobile with low affinities to soil.

  • NpO2(OH)2
  • NpO2(CO3)
  • NpO2(CO3)23−
  • NpO2(CO3)35−


Uses

Precursor in Plutonium-238 Production

237Np is irradiated with neutrons to create 238Pu, an alpha emitter for radioisotope thermal generators for spacecraft and military applications. 237Np will capture a neutron to form 238Np and beta decay with a half life of two days to 238Pu.

\mathrm{^{237}_{\ 93}Np\ +\ ^{1}_{0}n\ \longrightarrow \ ^{238}_{\ 93}Np\ \xrightarrow[2.117 \ d]{\beta^-} \ ^{238}_{\ 94}Pu}


238Pu also exists in sizable quantities in spent nuclear fuel but would have to be separated from other isotopes of plutonium.

Weapons applications

Neptunium is fissionable, and could theoretically be used as fuel in a fast neutron reactor or a nuclear weapon. In 1992, the U.S. Department of Energy declassified the statement that Np-237 "can be used for a nuclear explosive device". It is not believed that an actual weapon has ever been constructed using neptunium. Calculations show that the critical mass is between 50 and 60 kg. As of 2009, the world production of Np-237 by commercial power reactors was over 1000 critical masses a year, but to extract the isotope from irradiated fuel elements would be a major industrial undertaking.

In September 2002, researchers at the University of California Los Alamos National Laboratorymarker created the first known nuclear critical mass using neptunium in combination with enriched uranium, discovering that the critical mass of neptunium is around 60 kg, showing that it "is about as good a bomb material as U-235." US officials in March 2004, planned to move the nation's supply of separated neptunium to a site in Nevada.

Other

237Np is used in devices detecting high-energy (MeV) neutrons.

History

At least three times discoveries of the element 93 were falsely reported, namely as Bohemium on one hand and Ausonium one another in 1934 and then as Sequanium in 1939.

Neptunium (named for the planet Neptune, the next planet out from Uranus, after which uranium was named) was first discovered by Edwin McMillan and Philip H. Abelson in the year 1940 in Berkeley, California. Initially predicted by Walter Russell's "spiral" organization of the periodic table, it was found at the Berkeley Radiation Laboratory of the University of California, Berkeleymarker where the team produced the neptunium isotope 239Np (2.4 day half-life) by bombarding uranium with slow moving neutrons. It was the first transuranium element produced synthetically and the first actinide series transuranium element discovered.

\mathrm{^{238}_{\ 92}U\ +\ ^{1}_{0}n\ \longrightarrow \ ^{239}_{\ 92}U\ \xrightarrow[23 \ min]{\beta^-} \ ^{239}_{\ 93}Np\ \xrightarrow[2.355 \ d]{\beta^-} \ ^{239}_{\ 94}Pu}


Occurrence

Trace amounts of neptunium are found naturally as decay products from transmutation reactions in uranium ores. Artificial 237Np is produced through the reduction of 237NpF3 with barium or lithium vapor at around 1200 °C and is most often extracted from spent nuclear fuel rods as a by-product in plutonium production.

2 + 3 Ba → 2 Np + 3


By weight, neptunium-237 discharges are about 5 % as great as plutonium discharges and about 0.05 % of spent nuclear fuel discharges.

Synthesis

Chemically, neptunium is prepared by the reduction of NpF3 with barium or lithium vapor at about 1200 °C, however, most Np is produced in nuclear reactions:

  • When an 235U atom captures a neutron, it is converted to an excited state of 236U. About 81 % of the excited 236U nuclei undergo fission, but the remainder decay to the ground state of 236U by emitting gamma radiation. Further neutron capture creates 237U which has a half-life of 7 days and thus quickly decays to 237Np.


:\mathrm{^{235}_{\ 92}U\ +\ ^{1}_{0}n\ \longrightarrow \ ^{236}_{\ 92}U_m\ \xrightarrow[120 \ ns]{} \ ^{236}_{\ 92}U\ +\ \gamma}


:\mathrm{^{236}_{\ 92}U\ +\ ^{1}_{0}n\ \longrightarrow \ ^{237}_{\ 92}U\ \xrightarrow[6.75 \ d]{\beta^-} \ ^{237}_{\ 93}Np}


  • 237U is also produced via an (n,2n) reaction with 238U. This only happens with very energetic neutrons.
  • 237Np is the product of alpha decay of 241Am.


Heavier isotopes of neptunium decay quickly, and lighter isotopes of neptunium cannot be produced by neutron capture, so chemical separation of neptunium from cooled spent nuclear fuel gives nearly pure 237Np.

Role in nuclear waste

Neptunium-237 is the most mobile actinide in the deep geological repository environment.This makes it and its predecessors such as americium-241 candidates of interest for destruction by nuclear transmutation. Neptunium accumulates in commercial household ionization-chamber smoke detectors from decay of the (typically) 0.2 microgram of americium-241 initially present as a source of ionizing radiation. With a half-life of 432 years, the americium-241 in a smoke detector includes about 5 % neptunium after 22 years, and about 10 % after 43 years. After the 432-year americium-241 half-life, a smoke detector's original americium would be almost half neptunium.

Due to its long half life neptunium becomes the major contributor of the total radiation in 10000 years. As it is unclear what happens to the containment in that long time span, an extraction of the neptunium would minimize the contamination of the environment if the nuclear waste could be mobilized after several thousand years.

Isotopes

19 neptunium radioisotopes have been characterized, with the most stable being 237Np with a half-life of 2.14 million years, 236Np with a half-life of 154,000 years, and 235Np with a half-life of 396.1 days. All of the remaining radioactive isotopes have half-lives that are less than 4.5 days, and the majority of these have half-lives that are less than 50 minutes. This element also has 4 meta states, with the most stable being 236mNp (t½ 22.5 hours).

The isotopes of neptunium range in atomic weight from 225.0339 u (225Np) to 244.068 u (244Np). The primary decay mode before the most stable isotope, 237Np, is electron capture (with a good deal of alpha emission), and the primary mode after is beta emission. The primary decay products before 237Np are element 92 (uranium) isotopes (alpha emission produces element 91, protactinium, however) and the primary products after are element 94 (plutonium) isotopes.

237Np is fissionable. 237Np eventually decays to form bismuth-209, unlike most other common heavy nuclei which decay to make isotopes of lead. This decay chain is known as the neptunium series.

References

  1. "Restricted Data Declassification Decisions from 1946 until Present", accessed Sept 23, 2006


Literature

  • Guide to the Elements - Revised Edition, Albert Stwertka, (Oxford University Press; 1998) ISBN 0-19-508083-1
  • Lester R. Morss, Norman M. Edelstein, Jean Fuger (Hrsg.): The Chemistry of the Actinide and Transactinide Elements, Springer-Verlag, Dordrecht 2006, ISBN 1-4020-3555-1.
  • Ida Noddack: "Über das Element 93", in: Angewandte Chemie 1934, 47, 653–655.


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